Modern RF LDMOS transistors are usable as the active device in RF power amplifiers operating at frequencies over a range from below 1 MHz up to more than 2 GHz; including the 900 MHz and 1.8 MHz bands used by cellular telephones. For example, the Motorola MRF282SR1 is rated to produce about 10 watts at up to 2 GHz when operated class A or class AB and the Motorola MRF21120 is rated to produce 120 watts in the same frequency range when operated class AB.
LDMOS RF power devices are typically N-channel enhancement mode MOS field effect transistors.
RF amplifiers using these LDMOS devices typically have an impedance matching and input coupling circuit that couples an input RF signal from an input terminal of the amplifier to the gate of the LDMOS device while providing DC isolation between the input terminal and the gate. They typically also have an impedance matching and output coupling circuit that extracts an output RF signal from the drain of the LDMOS device to the output terminal of the amplifier device, while providing DC isolation between the output terminal and the drain and providing a path for power to the drain.
RF amplifiers using these LDMOS devices typically also have a gate bias circuit that provides a DC component, or gate bias, to the gate voltage of the device. For Class A or Class AB operation, this gate bias voltage is selected such that with no RF input the LDMOS device conducts a desired quiescent current between its drain and source terminals.
LDMOS RF power devices have gate threshold voltages that are temperature dependent as well as subject to process variation. The bias voltage for operation at a constant quiescent current in a Class AB RF amplifier is therefore both temperature dependent and unique for each device. For example, the Motorola MRF282SR1 is specified to have a gate threshold voltage in the range of 2 to 4 volts, and to require a bias voltage during Class AB operation of somewhere between 3 and 5 volts in order to maintain proper quiescent current. Similarly, the MRF282SR1 requires a bias voltage of between 3 and 5 volts for Class A operation, where the bias required for Class A operation is greater than that for Class AB operation. For details of the MRF282SR1 please see the data sheet MRF282/D available from Motorola's Semiconductor Products Sector.
Prior-art biasing circuits use a resistive voltage divider, implemented as a potentiometer or a pair of well-chosen resistors, to set the bias voltage for the LDMOS transistor. These circuits may utilize either a forward-biased diode or a thermistor to provide temperature compensation.
With resistive divider bias circuits an adjustment step is needed during manufacture of the RF amplifiers. This adjustment step sets the ratio of resistances such that the generated bias voltage gives proper quiescent current for the specific LDMOS transistor.
In Class AB amplifiers comprising a pair of LDMOS devices, this resistor ratio should ideally be set separately for each device of the pair so that the bias voltage produces proper quiescent current in each device. Alternatively, extra quiescent current may be carried to ensure linear operation despite minor variations in device characteristics.
As an LDMOS RF transistor ages, it is subject to hot-electron degradation. For N-type LDMOS devices, hot electron degradation causes an increase in the bias voltage required to maintain a constant quiescent current.
In amplifiers having resistive divider bias circuits, as the LDMOS power transistor ages hot electron degradation will cause quiescent current to fall off with time unless the bias circuit resistor ratio is readjusted. This falling-off of quiescent current may exceed ten percent. Prior amplifiers have required adjustment or have carried extra quiescent current so as to provide margin for hot-electron degradation. This extra quiescent current is undesirable as it causes inefficiency and thereby harms the environment.
It is advantageous to have a bias circuit that does not require adjustment to compensate for process variation or for hot-electron degradation over the life of the amplifier.
Most LDMOS RF power transistors are made as a plurality of smaller transistors, or cells, all fabricated on the same die but linked electrically in parallel. Each cell has a source, a gate, and a drain.
It is known in the art that transistors of the same type and dimensions fabricated together on the same die have similar initial device characteristics, thereby forming a matched pair. Similarly, ratioed matched pairs, or pairs of devices having similar gate threshold voltages but having drain currents differing by a known ratio N/M, are commonly built by paralleling N cells for a first device in the pair and M cells in the second device of the pair.
It is also known that with a ratioed matched pair fabricated on the same die and operating at the same drain voltage, not only are initial gate thresholds similar and the quiescent currents held to ratioed values, but the temperature and thermal coefficients and hot-electron aging characteristics of the devices tend to matched as well.